Many electronic display devices that we are familiar with are emissive, for example laptop screens, desktop monitors and televisions. Such devices, whilst highly visible indoors, are hard to see in bright (particularly outdoor) conditions. Additionally, such devices are often power-hungry due to the need to generate light either via a backlight in the case of liquid crystal displays (LCDs) or within the display itself in the case of CRTs or OLEDS.
An alternative to such emissive displays is to use a reflective display, in which the image is generated by modulating the intensity of the reflected ambient light. Such displays have the advantage of working with (rather than competing against) the strength of the ambient light, and hence are a good solution for displays which are used primarily in bright conditions. They also tend to have much lower power consumption, because there is no light generation involved. If necessary, the display can also be fitted with a front-light (which provides illumination from the front surface of the display) so that it can be read in darker environments also.
LCDs are notoriously inefficient when used in either transmissive or reflective mode, due to the losses involved in the polarisers, colour filters and black mask in the display structure. When used in transmission, a high brightness display can nonetheless be achieved by using a bright backlight, but this comes at the cost of high power consumption. When used in reflective mode, however, these inefficiencies result in a low reflectivity and as a result poor image brightness, much less than the brightness that would be achieved from the ultimate reflective display: paper.
In recent years, alternatives to reflective LCDs have emerged onto the market-place, the most commonplace being E-Ink technology. This has been used primarily to make monochrome e-book readers, such as the Amazon Kindle®. The white state reflectivity of such e-books is around 35-40% which, whilst comparable with newspaper, still falls short of the reflectivity from a quality white piece of paper. A more recent emerging technology is the Mirasol® display from Qualcomm MEMS Technologies, which uses an interference-based MEMS method to generate a switch between pixels which appear either black or green. Qualcomm claim 45% reflectivity in these displays which they term “bichrome” because the bright state is green rather than white.
These emerging technologies, whilst perhaps beginning to out-perform monochrome active-matrix LCDs, are still not reflective enough to be able to generate a high reflectivity colour image. To create a colour image, E-Ink would simply need to add colour filters to their display, which would cut the white state reflectivity down by about ⅓ to around 11-13%. Qualcomm claim to have a colour version of their Mirasol® display which works not by having colour filters as such, but nonetheless by having colour sub-pixels which (in their bright state) reflect either red, green or blue, but not tunable colour. Their projected white state reflectivity for such displays is 25%, which would be the highest reflectivity colour display on the market today.
However, whilst this performance is impressive, it still falls rather short of the white state reflectivity we are accustomed to in printed colour images on paper (˜60-70%), and is a direct result of the spatial sub-pixel method used to generate colour. An alternative is to use the interference-based MEMS pixels to generate tunable colour pixels, which would immediately triple the colour reflectivity simply due to the amount of area which is reflecting the correct colour. The issue is that colour tunable pixels, whilst in principle possible, often lack a good broadband white state and hence it is impossible to create a white state whose reflectivity is in proportion to that of the coloured states.
U.S. Pat. No. 5,835,255 (Miles; November 1998) describes an interference-based MEMS (IMod) display in which one type of pixel can switch between reflecting either none or some of the visible spectrum (therefore appearing either black or coloured in reflection). There is also another type of pixel described which can reflect either all or some of the visible spectrum (appearing either white or coloured). There is no type of pixel described which can reflect either none, some or all of the visible spectrum (appearing black, coloured or white). Some other types of pixels are described which can continuously tune their colour by being under analogue (rather than bistable) control.
U.S. Pat. No. 5,986,796 (Miles; November 1999), in addition to the material of the previous patent, describes another type of pixel which can switch between reflecting either none or all of the visible spectrum (appearing either black or white). However, it is not taught how to make this type of pixel. Neither can that type of pixel also reflect only some of the visible spectrum (appearing coloured).
U.S. Pat. No. 6,055,090 (Miles; April 2000) describes a reflective IMod display using the type of pixels previously described in U.S. Pat. No. 5,835,255 that can switch between reflecting either none or some of the visible spectrum (appearing either black or coloured). A method of making a colour reflective display is described in which the colour is generated by mixing light reflected from red, green and blue sub-pixels, rather than tuning the colour reflected from a single pixel. A red sub-pixel can reflect either no light or red light (appearing black or red). A green sub-pixel can reflect either no light or green light (appearing black or green). Likewise a blue sub-pixel can reflect either no light or blue light (appearing black or blue). A white reflection is generated by switching each of the colour sub-pixels to their colour reflecting state, and a white colour is perceived from the mixture of red, green and blue light. The brightness of the white state is limited by the fact the area of the display is divided into colour sub-pixels, because each part of the area of the display will always absorb at least part of the visible region of the spectrum.
U.S. Pat. No. 7,372,613B2 (Chui; May 2008) describes a reflective IMod display which again generates colour by using red, green and blue sub-pixels, as described above. It is explained how there is a trade-off between colour saturation and white state brightness. A method of switching each colour sub-pixel between states which appear black, coloured and white is described. The purpose of the extra white state is to boost the brightness of the display in the white state. The possibility of tuning the peak wavelength of the reflected light in the coloured state is not disclosed: colour is still generated spatially using red, green and blue sub-pixels.
U.S. application Ser. No. 12/568,622 (Sharp Laboratories of America, filed 28 Sep. 2009) describes a reflective IMod display which generates tunable colour through analogue voltage control via a buried ITO electrode, but there is no method described which would generate a white state whose reflectivity is in proportion to that of the coloured states generated.